The intruder was identified by his father as Aaron Barschak, 36, a comic who has staged at least one other similar prank and who calls himself a "comedy terrorist."
Fred Barschak, 72, said his son was probably trying to publicize his act by gatecrashing the Windsor Castle party Saturday night.
"He would not have been trying to harm anyone, not at all. He is actually not at all anti-royal," Fred Barschak said outside his north London home. "He is a professional standup comedian who is desperate to be a serious actor."
Police said Barschak was unarmed when he was detained. He was arrested on suspicion of burglary, which covers unauthorized entry to premises, and remained in custody.
Media reports said the comedian stumbled on to the stage where William was making a speech to thank his father, Prince Charles, and his grandmother, the queen, for organizing the "Out of Africa" themed party.
Barschak grabbed the microphone and began shouting before security officers dragged him away.
Earlier this year, he grabbed a microphone from London Mayor Ken Livingstone at an anti-war rally, telling the crowd he was a "comedy terrorist" and commenting on London traffic problems.
Home Secretary David Blunkett, the minister responsible for domestic security, demanded an immediate investigation into security procedures at Windsor Castle.
Sky Television showed Barschak — wearing a pink dress, turban, sunglasses and false beard — dancing outside the castle as guests dressed as characters out of Africa arrived.
The footage shows him shouting "Happy birthday. Out of Africa, I'm out of this world," before publicizing his performances at the Edinburgh Fringe Comedy Festival in August.
William, who is teaching himself Swahili and has been on safari in Africa several times, had chosen the "Africa" themed costume party because he wanted to avoid a "sterile" official function.
Guests included several members of the royal family, the prince's university friends, and celebrities such as comedian Rowan Atkinson, better known as the character "Mr. Bean."
In the latest of a series of interviews to mark his birthday, William crushed newspaper speculation that he does not want to become king and said he views his grandmother as an inspirational role model.
"All these questions about 'do you want to be king?' It's not a question of wanting to be, it's something I was born into and it's my duty," the prince, second in line to the throne, told the British news agency, Press Association in an interview released Sunday.
"But those stories about me not wanting to be king are all wrong. It's a very important role and it's one that I don't take lightly."
In the weeks building up to his birthday, newspapers had speculated that "Wills" — as he is known to family and friends — would shun his royal duties and intended to move to New York on graduation.
William is viewed as the modern face of British royalty — a sporty young man who dresses casually in jeans, shops and cooks for himself at university and enjoys a pint of cider with friends.
He has opted not to use the title His Royal Highness as "I don't want all the formalities because they're not needed for the time being."
But he revealed his admiration for the queen and the virtues he hopes to emulate and bring to the role of monarch.
"It's all about helping people and dedication and loyalty which I hope I have, I know I have," said William, who posed for the renowned fashion photographer Mario Testino to mark his birthday.
"I think particularly nowadays the monarchy plays a very important role. You only have to look at my grandmother and see the amazing things she's done," he said.
"That to me is a huge inspiration. The monarchy is something that needs to be there, I just feel it's very, very important. It's a form of stability and I hope to be able to continue that."
In recent years, the Windsor family has proved itself responsive to the demands of modern society. Following a bout of public criticism about the cost of the monarchy, the queen agreed in 1993 to pay taxes and has cut back expenses.
The royals have also learned to cope with their lives — including marital problems, divorce and death — forever being played out on the front pages of the newspapers.
But since the death of his mother, Princess Diana, William and his brother Harry have been insulated from intense press coverage. Editors have grudgingly agreed to respect their privacy, in return for official photo calls and interviews on key occasions.
William said the deal worked well and hoped it would continue.
"I don't think either side wants to return to the free-for-all of the old days. It's a really fine balance and it could be quite volatile if things get out of hand," he said.